In this specification the term "optical" is intended to refer to that part of the electromagnetic spectrum which is generally known as the visible region together with those parts of the infra-red and ultraviolet (UV) regions at each end of the visible region which are capable of being transmitted by dielectric optical waveguides such as optical fibres.
The present invention is based on the use of a plurality of gratings recorded in an optical fibre, the gratings having different Bragg wavelengths selected to store individual bits of data. The invention has particular application to optical fibre communication networks.
Advantageously, the optical memory further comprises read means for determining which, if any, fibre gratings are present. Other aspects of the present invention are as recited in the claims of this application, and are directed to optical communications systems incorporating an optical memory according to the present invention.
K. O. Hill, Y. Fujii, D. C. Johnson and B. S. Kawasaki first reported permanent, optically-induced changes of the refractive index of optical fibres in a paper entitled "Photosensitivity in optical fiber waveguides: Application to reflection filter fabrication." In their experiment, coherent radiation at 514.5 nm, reflected from the fibre ends, generated a standing wave in the fibre which induced a periodic refractive index change along its length. This formed a high reflectivity Bragg grating in the fibre which has a peak at the wavelength of the incident beam. Since then, numerous studies into the grating growth mechanism and photosensitive fibres have been carried out. However, the mechanism which results in the perturbation to the refractive index of the fibre core is not fully understood. The spectral region where the fibre is photosensitive has been found to range from the UV to around 500 nm.
B. S. Kawasaki, K. O. Mill, D. C. Johnson and Y. Fujii in an article titled "Narrow-band Bragg reflectors in optical fibres", Optics Letters Vol 8 No. 2 August 1978, pp 66-68 noted that an important property of the grating formation process is the extent to which the filter response can be tailored. For example, one method of forming a complex filter is to superimpose two or more simple band-stop characteristics in the same fibre by illuminating the fibre with different wavelengths of light either simultaneously or consecutively.
An alternative method forming two Bragg gratings is to apply a different longitudinal stress to the fibre before optically writing each grating, all the gratings having the same Bragg condition at the time of writing.
This method of writing two or more gratings has the advantage of obviating the need for multiple wavelength illumination.
The property used in this method is to exploit the fact that optical fibre can theoretically be linearly strained by up to 20%. If a photosensitive fibre, length 1, is illuminated by a light from a laser of wavelength .lambda..sub.0, this will result in a grating of period about .lambda..sub.0 /2n.sub.eff, where n.sub.eff is the fibre mode refractive index. If the fibre is now stretched by .DELTA.1; then, when illuminated, a grating of similar pitch to before will be written. By allowing the fiber to relax to its normal length, the pitch of this second grating will now be slightly smaller than the first grating. For the case of a reflection filter, the second grating has a peak wavelength which is smaller than the writing wavelength.
It may also be possible to write reflection gratings for use at the telecommunications bandwidth of 1.3-15 .mu.m, if the fibre can be stretched by approximately 10%. This is still within the theoretically predicted change; but, due to defects in the manufacture of the fibre, it is not clear whether it is possible to do this. Other materials, such as photorefractive materials, may also be used as a data storage medium for the purposes described here.
A convenient method of applying the different longitudinal stresses to the fibre to produce the different stains is to clamp one end of the fibre, and to apply the stress by means of a piezo-electric translation stage clamped to the other end of the fibre. Clearly, other stressing means may be used.